dm9000: Support MAC address setting through platform data.

The dm9000 driver reads the chip's MAC address from the attached EEPROM. When
no EEPROM is present, or when the MAC address is invalid, it falls back to
reading the address from the chip.

This patch lets platform code set the desired MAC address through platform
data.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>